U.S. Pat. No. 5,631,346 describes a process for degrading polyolefins in a twin-screw extruder operated at a pressure of 1-100 bar, wherein the pressure is changed with a range of variation of 0.5-30 bar. A dissertation entitled "Studies of Thermal/Peroxide Induced Degradation and Maleation of Polypropylene in a Modular Intermeshing Twin Screw Extruder" is indicated to have been presented by Byoungjoo Kim at the University of Akron in May, 1996 wherein the effects of screw configurations, screw speeds and peroxide concentrations were examined for both co-rotating and counter-rotating twin-screw extruders at temperatures of 230.degree. C. up to a maximum of 290.degree. C.
British Patent No. 1,042,178 describes narrowing the molecular weight distribution of polyolefins by degradation using high shear gradients at temperatures between the melting point and the temperature at which purely thermal degradation of the polyolefin occurs.
U.S. Pat. No. 3,087,922 describes a process for the pyrolysis of a high molecular weight polyolefin to produce a lower molecular weight polymer by passing the polymer through a pyrolysis zone in the form of a fluid stream having annular cross section, and transferring heat of pyrolysis to the polymer through both exterior and interior boundaries of the annular stream. U.S. Pat. No. 3,248,469 describes a method of visbreaking a polymer of an olefin by feeding the polymer to a shearing zone at a controlled rate, passing sheared polymer through an extrusion zone, and maintaining a positive back pressure on the polymer. U.S. Pat. No. 5,292,862 describes a continuous process for the production of low molecular weight polyolefin by feeding the polymer to an extruder for melting, metering the molten polymer at a constant rate to a tubular degradation reactor, and thermally degrading the polymer in the reactor in the presence of a higher fatty acid, a hydrochloric acid acceptor or a mixture thereof.
The following patents describe polyolefin degradation processes that utilize a catalyst. U.S. Pat. No. 3,332,926 describes a process for degrading polyolefins by mixing a crystalline polyolefin with a metal salt of a carboxylic acid and heating the resultant mixture in an atmosphere substantially free of oxygen to a temperature of 275-450.degree. C. U.S. Pat. No. 3,345,352 describes a process for degrading polyolefins by heating the polymer to a temperature of 275-450.degree. C. in the presence of an oxide or carbonate catalyst and in the absence of oxygen for at least five minutes. U.S. Pat. No. 3,519,609 describes a process for degrading polyolefins at temperatures of 200-400.degree. C. in the absence of oxygen and in the presence of an organic anhydride catalyst. U.S. Pat. No. 3,562,788 describes a process for degrading polyolefins in the presence of metal coordination polymerization catalyst residue. U.S. Pat. Nos. 3,723,402 and 3,842,153 describe processes for degrading isotactic polypropylene in the presence of an organosilicon compound.
The following patents describe processes for degrading polyolefins wherein an oxygen-containing gas is utilized. U.S. Pat. Nos. 3,551,943; 3,563,972; and 3,608,001 describe degradation processes utilizing an extruder-reactor to heat mixtures of polypropylene with an oxygen-containing gas, wherein feedback means are provided to change the temperature in the extruder thereby controlling the molecular weight of the exiting polymer. U.S. Pat. No. 3,898,209 describes a process for controlled scission of polypropylene by injecting, under pressure, controlled amounts of oxygen into the melt-phase of the polypropylene at specified temperatures. U.S. Pat. No. 3,940,379 describes a process for controlled oxidative degradation of propylene polymers by injecting oxygen or an oxygen-containing gas and an organic or inorganic peroxide, melting and working the resulting mixture in a high shear zone, and recovering an essentially odor-free propylene polymer having a melt flow rate higher than that of the feed polymer.
The following patents describe processes for degrading polyolefins by mixing another polymer therewith. U.S. Pat. No. 3,121,070 describes the degradation of polypropylene at temperatures of 275-450.degree. C. in the presence of a modifying polymer. U.S. Pat. No. 3,598,885 describes a process for preparing low molecular weight polymers by mixing relatively thermally stable polyolefins with relatively thermally unstable polymers and thermally cracking the mixture.
The following patents describe processes for degrading polyolefins wherein a free-radical initiator is utilized. U.S. Pat. No. 4,707,524 describes the degradation of polypropylene by mixing a peroxide therewith and melting the resulting mixture under thermal mechanical action. U.S. Pat. No. 4,749,505 describes a process for degrading polyolefins in the presence of a free-radical initiator, under an inert blanket of nitrogen. U.S. Pat. No. 5,594,074 describes a process wherein unreacted free-radical generators are used to produce polymer pellets that are degraded by thermal treatment. U.S. Pat. Nos. 3,862,265 and 4,001,172 describe processes for degrading polyolefins in extruders wherein free-radical initiators are injected into the reaction zone. U.S. Pat. No. 5,530,073 describes a process for the controlled degradation of polypropylene having increased peroxide efficiency. U.S. Pat. No. 5,587,434 describes a process for degrading polypropylene wherein polypropylene is mixed with a free-radical generator in increments, in an extruder. This is also described in U.S. Pat. No. 3,144,436.